Environment variable for downloading Android package

Android artifact directory name may not match package release name. This change adds flexibility to allow for the difference. The corresponding change has already been made to the Jenkins job.

Change-Id: Ib990e9c53843773a5787cf223221f106a2c6a837
Reviewed-on: http://review.couchbase.org/33538
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>